In media handling assemblies, particularly in printing systems, accurate and reliable registration of an image as it is transferred is desirable. In particular, accurate registration of an image as it is transferred to a target substrate media or an intermediate transfer belt has a direct correlation to image quality.
Contemporary media handling assemblies use controllers, in the forms of automated processing devices, in order to maintain control of the belts and/or sheets they are handling. Often that control is maintained by adjusting a belt velocity which conveys marking material and/or sheets of paper on transfer belts to a transfer station. Also, steering systems are used in order to guide and/or control the position of the belts along a process direction. While controlling the belt speed along the process direction is important, maintaining lateral control and stability can also be a challenge.
Many belt or web transport systems employ an actuator, in the form of a tiltable steering roll, in order to control lateral movement. The rotational axis of the steering roll is tilted in order to encourage the belt supported thereon to laterally shift. After receiving input from sensors measuring the speed and position of a belt, a controller executes a command profile in order to adjust the belt position when it has drifted from its base position. Such systems are particularly common in printing systems, but are also found in other substrate media handling assemblies.
Typically, control systems are employed to manage the operation of the marking device. Additionally, the control systems attempt to manage most disturbances that result in motion errors and thus effect image quality. One example of such control techniques is a closed-loop technique that uses feedback control. Feedback control reacts to disturbances by attempting to correct for them on the next system cycle. Closed-loop feedback control is similar to a trial-and-error based system; making a correction, measuring the results and then further correcting on the next go-around. However, as feedback control is reactionary, it tends to lag in its response and thus may not compensate fully for quick or transient disturbances.
Another known control technique is feedforward control, which uses an open-loop system that accumulates information for future use based on prior calibration and/or preliminary setup of the system. The feedforward control can eliminate the response lag and anticipate known system disturbances. However, belt steering actuators, while useful to reposition a belt, cause an induced disturbance to the belt, which can lead to belt position tracking errors. Such induced disturbances combine with the fact that most endless-loop belt systems have belt edges that are not straight and/or have inherent movement profiles that are not perfectly straight. Accordingly, attempts by feedforward control to manage a combination of inherent movement and steering induced movement errors lead to overcompensation, which translates to position errors of images.
Accordingly, it would be desirable to provide an apparatus and method capable of more accurately reducing registration errors in a media handling assembly, and thereby overcomes the shortcoming of the prior art.